Dear Gert:

Goro Miyamoto did some work by indexing martensite with different tetragonality, and I am not sure he has published their result or not. You can write to him ([email protected]). Or I can ask him since I will meet him in a recent conference.

Xinfu

Hello Gert,

We did indexed low-tetragonality martensite with BCC patters. The measured tetragonality was 3% or lower resulting in successful indexing with BCC patterns. Higher teragonalities may lead to lower confedence index, higher pattern misfit etc. They are still indexable with BCC as in the case of 0.5% Carbon steels. Indexing sucuess rate and pattern quality would significantly drop due to larger pattern misfit and higher dislocation density.

Hope this helps.

Majid

Hi Majid,

thanks for this information. You did not publish these results somewhere?

Could you show IPF maps with tetragonal indexing? Simply from your terminology I assume you did it with a OIM, right?

Dear Gert,

We tried indexing Martensite in low carbon steels using the body-centered tetragonal (BCT) phase, but in practise this doesn’t work with regular indexing, i.e. in Dual Phase steel (Ferrite/Martensite) the phases are not distinguished when we try to index with BCT and BCC. However, we usually use the lower Image Quality to discern the martensite in the DP steels, using just the BCC phase.

Possibly with Dictionary Indexing it would be more likely to work?

As far as I understood the dictionary approach it preferably works for patterns where Hough transformation does not deliver correct band positions anymore. Disadvantage: 1) Like in ASTAR you only have a certain amount of simulated patterns for comparison (the size of the dictionary). 2) You do not have the lattice parameters. c/a is practically unknown for martensite, except you use the carbon concentration for a rough estimation, or you have XRD results and use the assumption that the visible peaks describe the lattice parameters a and c, right?

Did anyone use the advanced indexing of Oxford?

How accurate are the lattice parameters in martensite?

Tijmen Vermeij actually in dual phase steels usually there is so little carbon you shouldn’t observe tetragonality.

Gert

Welcome. Quenched L80 drill pipe steel (C

Try indexing with lowest grain size in range of 0.25 microns and high grain boundary mismatch around 5-6. Or else try CI indexing around 0.1 correction.

@ M. Khan: I did not ask how to measure martensite but whether somebody indexed martensite as tetragonal description. Successfully... As far as I know, nearly everybody indexes martensite as cubic ferrite. In a postprocessed step one tries to find a correlation between "bad" patterns and martensite, but this might be misleading.

Hallo Gert,

there are 2 kinds of martensite. Normally, our martensite is allways cubic (bcc). This is independent of the C-content. We analyze it with XRD, where you can see a slightly higher lattice constant. This martensite can not be distinguished via EBSD-pattern recognition. You see it only in the band contrast or even better in the band slope contrast. Martensite is dark and ferrite ist bright. I think you know this.

But there exists a different kind of martensite. I have seen it only in medium manganese steels which contain also aluminium. Maybe it appears also in stainless steels. But here we do not have such steels. You see in the XRD spectrum a tetragonal phase. We put the measured lattice parameters into the channel5 programm and define our own phase. This phase can be recognized by aztec and can easily be mapped, because the crystal structure is different. If you are interested I can upload a phase map.

Good idea. Could you upload the diffractogram (how big was your tetragonality?) as well as the EBSD phase and at least one IPF map?

How different the structure of your martensite is compared to bcc? Or do you mean the structure "description" by space-group type?

I asked my colleague Christian Commenda. He has made measurements which where published in a paper: Materials Science & Engineering A 735 (2018) 475–486, written by Simone Kaar et al. So these data are published anyway and I do not get trouble for posting secrets.

In the phase map is blue bcc ferrite, red fcc austenite and green bct martensite.

In the XRD comparison the gray 00_a curve shows the tetragonal phase.

I think we took the data shown in the pdf for defining the martensite.

I forgot to upload IPF-maps.

Dear colleagues,

As Martin already mentioned, the XRD study of the shown EBSD results of the sample M1160400 was based on the information given in the data base file PDF-00-044-1289.

I would like to add a Rietveld refinement of the measured XRD data to get the lattice parameters for the tetragonal martensite a=2.8449, c=3.0430, which gives a tetragonality c/a=1.0696. The overall chemical composition in weight percent is 0.4C-2.5Mn-5.2Al and the phase fraction from Rietveld refinement is equal to 8wt%.

It would be nice to deform or cool down the sample and see which phases come out from the retained austenite. For higher manganese values and lower aluminium values we get epsilon martensite (hexagonal), which can also be nicely destinguished from the bcc and fcc phases in EBSD.

Best wishes,

Chris

p.s.: The added XRD plot depicts the Rietveld fitting result.

Hi Christian, most important of such plot is the difference intensity. Could you plot this as well. Moreover, I do not see any noise. Did you smoothed the diffractogram?

I have to admit that I was more speculating on tetragonalities clearly smaller than 7% :-). Anyway, thanks for the contribution.

Hi Gert,

You may find this recent M&M publication helpful.

High Angular Resolution Electron Backscatter Diffraction Studies of Tetragonality in Fe-C Martensitic Steels, Volume 24, Supplement S1 (Proceedings of Microscopy & Microanalysis 2018) August 2018, 962-963

Link to Abstract:

https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/high-angular-resolution-electron-backscatter-diffraction-studies-of-tetragonality-in-fec-martensitic-steels/141568ACC2FF46D91D1C8EF8C7D2B152

PDF:

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/141568ACC2FF46D91D1C8EF8C7D2B152/S1431927618005305a.pdf/high_angular_resolution_electron_backscatter_diffraction_studies_of_tetragonality_in_fec_martensitic_steels.pdf

Dear M. Abbasi,

I knew this work, thanks. Unfortunately it is done with HR-EBSD, i.e. you need a lot of time and memory to do this investigations which also explains the compararively small area investigated.

On the other hand, it suggests an interesting observation: Martensite has no single c/a ratio but a c/a distribution? Is this reasonable? Another problem: I do not see any scale bar describing the real size of the map... :-( So it is impossible to judge whether the variation in carbon concentration is reasonable? How did they measure it?